Some applications of trailer refrigeration units have special problems associated with defrost. A trailer that is used to transport and deliver ice cream, for example, has special problems. It is desirable to maintain the ice cream at -20.degree. F., or below. Even if the product is loaded at the correct temperature into a pre-cooled trailer, water vapor in the air, which enters the trailer during the loading process, will freeze on the fins of the evaporator coil. In addition, each time the trailer door is opened during a delivery, warm, moist air will enter the trailer adding to the amount of ice forming on the evaporator coil. Frequent deliveries on a hot humid day can result in many defrost cycles, with a considerable amount of water being melted off the evaporator coil.
Because the surface of the evaporator pod beneath the coil is very cold, the defrost melt dripping off the coil fins refreezes on the pod before it can run down into the drain pan. The resulting ice can build up to the point where it begins to restrict the flow of air returning to the coil. The ice can also sublimate to vapor during the cooling cycle, where it immediately refreezes back on the coil, requiring more defrosts.
In order to maintain load temperature and product quality, it is desirable to keep the length and frequency of the defrost cycles to a minimum. In the past, a hot gas line has been located in the drain pan to keep the drain holes free of ice. While this does keep the drain pan free of ice, it does nothing to prevent ice from building up on the evaporator pod